1. Field of the invention:
This invention relates to a pressure device for use in image forming apparatuses in which a media sheet that has been exposed to light is pressed together with an image receiving sheet to reproduce an image on the image receiving sheet.
2. Description of the Prior Art:
An image forming method is known in which a media sheet with a selectively hardened image formed thereon by photosensitive microcapsules exposed to image light is pressed together with an image receiving sheet coated with developer, whereby unhardened photosensitive microcapsules contained in the media sheet are ruptured, causing encapsulated colorless dyes to flow out and react with the developer to reproduce an image on the image receiving sheet that corresponds to an image of the original. Generally, in an image forming apparatus using such image forming method, a pair of rollers are used as a pressure device for pressing the media sheet together with the image receiving sheet. Since a line pressure of about 90 kg/cm is needed for the pressure device to rupture the unhardened photosensitive microcapsules on the media sheet, the pair of rollers are pressed against each other with a large pressure applied to the ends thereof so as to achieve the required line pressure on the contact portion that is formed parallel with the axial direction of the rollers. On the other hand, to maintain good conditions for the formation of an image on the image receiving sheet, the pressure device must be so constructed that the media sheet is pressed against the image receiving sheet evenly over the entire surface thereof to prevent variations in the copy image density. This means that a uniform line pressure must be applied over the entire contact portion of the pair of rollers. Various ideas have been employed with regard to the roller materials, shapes and other structural improvements, such as the provision of a lower roller with a hollow construction, so that the line pressure acting on the rollers, the ends of which a large pressure is applied, to be kept within the allowable limits to prevent the deflection of the rollers.
However, conventional pressure devices for an image forming apparatus have been so constructed that sheets with a width corresponding to the length of the entire contact portion of the rollers pass through the said entire contact portion and that a designated line pressure is obtained over the entire contact portion of the rollers, which makes it difficult to handle an increase in the line pressure due to the stress concentration caused when an image receiving sheet of a different size is passed between the rollers. That is, as shown in FIG. 7, when an image receiving sheet 73 of a smaller size passes between the upper and lower rollers 71 and 72, the force F that is applied to the ends of the rollers 71 and 72 acts on the upper and lower rollers 71 and 72 along the length perpendicular to the transporting direction of the image receiving sheet 73, thus resulting in an increased line pressure per unit length. This may cause the line pressure acting on the rollers 71 and 72 to exceed the maximum design limit (the allowable limit within which deflection is not caused), deflecting the rollers 71 and 72 with the sides of the image receiving sheet 73 as the fulcrums. The deflection prevents the contact portions of the upper and lower rollers 71 and 72 from aligning parallel with each other, resulting in a smaller line pressure acting perpendicularly to the image receiving sheet 73 and media sheet 74 on the areas adjacent to the sides of the image receiving sheet 73 as compared with the center areas thereof, thus preventing uniform line pressure from being applied over the entire surface of the image receiving sheet 73, which causes variations in the copy image density. The conventional pressure devices have had a further problem in that, because the line pressure acting on the image receiving sheet 73 and media sheet 74 is biased (with respect to the direction along the surface), a force in the direction along the surface is caused as a component force of the line pressure, thus creating creases in the media sheet 74, as well as the image receiving sheet 73.